1. Technical Field
The present invention relates to a method for a safety valve and a safety valve in the form of a valve body displaceable in a cavity with an inlet on either side of which are arranged outlets, whereby liquid and/or gas flow takes place from the inlet via a flow path passing through the valve body to the outlets. Alternatively, there can be two inlets and an outlet arranged between them, whereby liquid and/or gas flow takes place from the inlets via a flow path passing through the valve body to the outlet.
2. Description of the Background Art
Safety valves of this type exhibit a cavity with a cylindrical shape in which a plunger moves axially. The valve is provided with an inlet to the cavity as well as an outlet from the cavity arranged on either side of the inlet. Depending on the position of the plunger in the cavity, the plunger seals one or the other of the outlets, or allows both to be open. A safety valve of this type thus permits flow through two separate pipes connected to the outlets over the distance where the risk of a break in the pipe is considered to merit extra security. When a break in a pipe occurs in one of the separate pipes, the pressurized medium, liquid and/or gas, is nevertheless continuously maintained via the second pipe in all significant respects with regard to flow and pressure. As the pressure at the outlet to the damaged pipe falls in relation to the pressure prevailing at the outlet to the second pipe, the plunger will be displaced towards the outlet with the lower pressure and seal this outlet.
The disadvantage of these tube-breakage valves, is however, that their function is based on the difference in pressure that is expected to arise when the pipe breakage occurs, but not on the forces of mass seen as a result of the increased flow that act on the valve body in a direction against the “remaining pipe”. These forces of mass can bring about that the effect of the pressure difference that has arisen ceases and the valve does not attain that position in which it stops the unwanted leakage flow. The valve body does not react to small flows but rather requires that the break in the pipe brings about a significant leakage flow in order for the valve body to close. Another problem with this type of valve is that when the valve is brought into action and one pipe is blocked, (which can often happen spontaneously at start-up), the valve will, depending on the difference in pressure created, remain in position. If, in this case, the active pipe is damaged, it is highly likely that the leakage in this pipe will not be prevented, which means that the desired safely function is not achieved. In addition, there is no dampening function, which can initiate self-oscillation of the valve body.